FIG. 1 illustrates a prior art steam generator 10. The steam generator 10 includes a shell 12, a tube bundle 14 positioned within the shell 12, a tube sheet 16 supporting the tube bundle 14, and a water box 18 positioned beneath the tube sheet 16. The tube bundle 14 includes a set of "U" shaped tubes 20. For clarity, only one "U" shaped tube 20 is shown in the figure, but it should be understood that the tube bundle 14 can be made up of thousands of individual tubes 20. A division plate 22 divides the water box 18 into an inlet section 24 and an outlet section 26.
The steam generator 10 receives hot reactor coolant in the water box 18 through nozzle 28. From the inlet section 24 of the water box 18, the coolant flows through tubes 20 to the outlet section 26 of the water box 18 and back to the reactor (not shown) through nozzle 30. Secondary water enters the shell 12 through nozzle 40 and is heated by contact with the tubes 20. As the secondary water is heated, it boils to generate steam, which exits the shell 12 at the top of the steam generator 10 through nozzles 42 and 44. The steam thus generated is routed to a steam turbine (not shown) where it is expanded to drive an electrical generator (not shown).
The tubes 20 are connected to a tube sheet 16 by seal welding or by expanding the tubes within the tube apertures in the tube sheet 16. Located at various heights in the shell 12 are tube support plates 50 containing apertures through which the tubes 20 pass. The apertures in the tube support plates 50 are slightly larger in diameter than the outside diameter of the tubes 20 so that the tubes can slide vertically within the support plates. This relative sliding capability is necessary to accommodate differential thermal expansion which occurs when the steam generator 10 is brought on-line and is slowly heated to operating temperature.
The steam generator tubes 20 are susceptible to several types of corrosion mechanisms that can ultimately lead to leakage or significant wall thinning. These corrosion mechanisms include primary water stress corrosion cracking, secondary side intergranular attack, secondary side intergranular stress corrosion cracking, and secondary side wastage. Primary side degradation typically occurs at locations of high tensile residual stress, such as expansion transition areas, inner row U-bends, and dented tube support locations. Secondary side degradation occurs at locations where impurities can concentrate, providing corrosion sites, such as tube-to-tubesheet crevices, tube support plate-to-tube interfaces, anti-vibration bars interfaces, and sludge pile regions (located at the tubesheet).
Current techniques to mitigate corrosion-induced problems include: plugging the degraded tubes, sleeving the degraded tubes, and replacing the steam generator. Plugging the degraded tubes takes the tube out of service, reducing the steam generator efficiency. The ability to plug tubes is based on the "plugging margin" that is calculated based on operating experience for each steam generator.
Sleeving is a more expensive mitigation technique; it involves welding a tube section or "sleeve" to the interior surface of a degraded region of the existing tube. Sleeving allows the tube to remain in service, with some reduction in flow rate. Sleeving is generally performed when the steam generator "plugging margin" is approached. The final option is to replace the steam generator. Replacement of the steam generator addresses the problem, but at a prohibitive cost.
The Electric Power Research Institute, the assignee of the present invention, has developed improved corrosion mitigation techniques that rely on cladding the inside surface of steam generator tubes with corrosion resistant weld metal using a nd:YAG laser to provide a permanent repair for steam generator tubing. This technology is described in U.S. Pat. Nos. 5,430,270 and 5,514,849, both of which are expressly incorporated by reference herein.
A majority of steam generator tube failures occur at the tubesheet region. During fabrication of the steam generator, tubes are mechanically or hydraulically expanded along the length of the tubesheet apertures to seal the area between the tube and the tubesheet. The intent is to prevent water and corrodants from creating corrosion products between the tube and the tubesheet. However, at the top surface of the tubesheet, a small change in tube diameter occurs (referred to as a "roll transition") where the rolling process is completed. This area is highly stressed and subjected to secondary water environment of the steam generator, which contains a variety of contaminants. Sludge from the accumulation of secondary water impurities further compounds the problem by providing additional concentration of corrosion products. As a result, this region is very susceptible to stress-corrosion cracking.
Currently, sleeving is the principal means of repair for tube-to-tubesheet location failures. However, recent plant experience has shown that the sleeves may fail due to improper installation, insufficient heat treatment, cold work stresses, and other factors. Consequently, techniques have been developed to cut and remove sleeves and to install new sleeves.
In view of the foregoing, it would be highly desirable to provide a method for repairing heat exchanger tubes. More particularly, it would be highly desirable to provide a method for repairing heat exchanger tubes at the tube-to-tubesheet region of a steam generator, which is the most common location for steam generator tube failures. Ideally, such a method would rely upon existing repair equipment.